1. Field of the Invention
The present invention relates to a computer system using memory in substitution of a hard disk drive, and more particularly, to a computer system using a dynamic random access memory in substitution of a hard disk drive.
2. Description of the Prior Art
Presently, microprocessor-based systems used for handling a great amount of data are applied in a broad spectrum of fields. The most commonly used system is a general computer system in an office or at home. When working, people can deal with digital data quickly and efficiently with the help of the computer system. When at home, people can use the computer system to entertain themselves with a multimedia program such as a computer game. Recently, the processing speeds of computer systems have greatly improved due to the advanced development of central processing units (CPUs). The operating frequency of the CPU has exceeded one gigahertz, that is, the total number of instructions executed within one second has greatly increased, thereby making the computer system more powerful. However, the computer system includes a plurality of components, and each component has its own processing speed. Therefore, components with a low processing speed will delay the overall performance of the computer system.
Please refer to FIG. 1, which is a block diagram of a first prior art computer system (10). The computer system (10) has a central processing unit (CPU) (12), a north bridge circuit (14), a south bridge circuit (16), a memory component (18), a hard disk drive (20), a video graphics array (VGA) card (22), a display device (24), and an input device (26). The CPU (12) is used for controlling each circuit in the computer system (10) to make the computer system (10) function normally. The north bridge circuit (14) is electrically connected to the CPU (12) for managing data transmissions between the CPU (12) and the memory (18) (dynamic random access memory for example), and managing data transmissions between the CPU (12) and the VGA card (22). The display device, such as a monitor, is electrically connected to the VGA card (22) for displaying video data outputted from the VGA card (14). The south bridge circuit (16) is electrically connected to the north bridge circuit (14) for exchanging data with the CPU (12) through the north bridge circuit (14). For example, the south bridge circuit (36) is capable of handling data transmission between the input device (26) and the CPU (12), and handling data transmissions between the CPU (12) and the hard disk drive (20). The input device (26) could be a keyboard or a mouse. As the memory (18) is a volatile storage medium and the hard disk drive (20) is a non-volatile storage medium, data stored in the memory (18) will be lost when there is a lack of electric power, but the data stored on the hard disk drive (20) are not affected by the lack of electric power when the computer system (10) is shut down. Therefore, user data are mainly stored on the hard disk drive (20) to prevent data from being lost due to a sudden shortage of electric power. The operation of the computer system (10) is briefly described as follows. The CPU (12) sends a data access instruction to the hard disk drive (20). The data stored on the hard disk drive (20) are retrieved and are transmitted from the south bridge circuit (16) to the north bridge circuit (14). The north bridge circuit (14) then stores data from the hard disk drive (20) on the memory (18). The CPU (12), therefore, can access data stored in the memory (18) through the north bridge circuit (14), and temporarily stores the retrieved data into a cache (not shown) of the CPU (12) to be further processed. When the computer system (10) prepares to shut down, data stored in the memory (18) will first be restored onto the hard disk drive (20) to prevent data stored in the memory (18) from being lost. However, the data access speed of the memory (18) is faster than that of the hard disk drive (20). For example, the data access speed of the memory (18), such as a dynamic random access memory, is in nanoseconds. However, the data access speed of the hard disk drive (20) is based in milliseconds. That is, the hard disk drive (20) is a peripheral with a slow processing speed. The performance of the computer system (10) is then deteriorated by the speed of the hard disk drive (20). In addition, concerning the amount of data transferred per second, the memory (18) is also better than the hard disk drive (20). Therefore, it is a logical step to increase the performance of the computer system (10) with the help of the memory (18) owing to the short access time and great transmission efficiency of the memory (18).
Please refer to FIG. 2, which is a block diagram of a second prior art computer system (30). The computer system (30) has a CPU (32), a north bridge circuit (34), a south bridge circuit (36), a memory (38), a hard disk drive (40), a VGA card (42), a display device (44), and an input device (46). The function of each element in the computer system (30) has been described above in the section related to the computer system (10) and there is no need to repeat the descriptions. The memory (38) has a system block (48) and a RAM driver block (50) in which the system block (48) is used by an operating system, and the RAM driver block (50) is used for temporarily replacing the hard disk drive (40). The memory address range and related capacity of the RAM driver block (50) are transmitted to the operating system during a power on self test (POST) process. The operating system will then store a terminate and stay resident (TSR) program (52) in the system block (48). The TSR program (52) is used for managing the data access of the RAM driver block (50). When the CPU (32) attempts to access the hard disk drive (40), the TSR program (52) first intercepts an interrupt vector related to access of the hard disk drive (40), converts a hard disk drive access instruction into a memory access instruction, and converts data complying with a hard disk drive storage format into data complying with a memory storage format. The data, which are originally designated to be recorded on the hard disk drive (40), are temporarily stored in the RAM driver block (50). As the data access speed of the memory (38) is faster than that of the hard disk drive (40), the CPU (32) uses the TSR program (52) stored in the memory (38) to manage the data access of the RAM driver block (52) so that the CPU (32) can quickly use data stored in the RAM driver block (52) to speed up the operation of the computer system (30). In addition, the CPU (32) restores data that has been processed into the RAM driver block (50) from the hard disk drive (40) with the help of the TSR program (52). In conclusion, the computer system (30) uses a software method, the TSR program (52), to manage data flow between the memory (38) and the hard disk drive (40) without modifying actual hardware circuits. However, the computer system (30) still requires the presence of a hard disk drive (40) because the memory (38) is a volatile storage medium. If the computer system (30) is turned off, the memory (38) will lose stored data owing to a sudden shortage of electric power. In other words, the computer system (30) has to restore data stored in the memory (38) into a non-volatile storage medium, such as the hard disk drive (40), to maintain the data when the system is shut down. This is the reason why the computer system (30) requires the presence of a hard disk drive (40).
Furthermore, the prior art computer system also adopts non-volatile memory such as flash memory as a storage medium for the computer system, and also uses the same technology of computer system (30) shown in FIG. 2 to store data without a hard disk drive. As the flash memory and the hard disk drive are non-volatile storage media, the data stored in the flash memory are maintained when the computer system is shut down. That is, the computer system with the flash memory does not need to restore data back to the hard disk drive when the computer system is about to shut down. However, the flash memory stores binary data by changing the threshold voltage of each memory cell thereof, and the time spent on writing data is longer than that of general dynamic random access memory. That is, the processing speed of the flash memory is slower than that of the dynamic random access memory. In addition, the cost of manufacturing the flash memory is greater than that of the dynamic random access memory when the same memory capacity is considered. Concerning the cost of such a product, the flash memory is not suitable for a computer system that must access a great amount of data.